The field of the present invention is the production of piston rings for use in internal combustion engines.
In conventional internal combustion engines, pistons are employed which typically include a circular transverse sectional shape. Piston rings used with such pistons are themselves circular and are easily formed. The exterior surface of such rings may be easily finished by simply rotating a stack of piston ring blanks about the axial centerline thereof while applying a cutting tool to the exterior surface.
Internal combustion engines have also been developed which employ cylinders and pistons located therein which are noncircular in transverse section. These cylinders are elongated in plan in order to accommodate the more efficient intake and exhaust valve arrangement providing greater effective port area. One such device is illustrated in U.S. Pat. No. 4,350,126, granted Sept. 21, 1982 to Shoichi Honda, the disclosure of which is incorporated herein by reference. The corresponding piston rings for such noncircular pistons are, of necessity, also noncircular, being oval, elliptical or otherwise of an elongated shape. Consequently, the exterior surface of such rings cannot be formed by the aforementioned conventional procedure.
In certain circumstances, piston rings have been used which employ an external peripheral surface that is conically tapered. The foregoing problem of not being able to easily machine piston rings of noncircular plan is compounded in that the ring blanks cannot be coaxially stacked in a single jig. Instead, they must be stacked with spacers in between each ring blank. Specialized rotary cutting tools having a series of precisely formed cutting teeth of conical shape have been employed. Such devices are illustrated in FIG. 7. Processing the piston ring blanks in this manner to achieve a conical exterior surface is disadvantageous because of the required effort to assemble and disassemble the blanks and spacers from the jig and because of the expensive tooling required.